The cables referred to in this invention are those which are constructed by means of applying in a helical manner successive layers of load bearing elements or wires around an initial center member or core. This core may itself be a wire or family of wires; a hydraulic hose or one or more electrically insulated conductors or the like. The lead bearing elements may be metallic wires, plastic fibers, or the like, and may may have round, square, z, or other shaped cross-sections. The finished cable may have one or many successive layers of helical elements applied in the same or opposite directions.
In forming cables of this type of construction, it is common to experience loosening or ballooning of the helical wires during manufacture of the cable and later when load is applied to the cable in use. This loosening is a very serious problem for the manufacturer and in the operation of the cable. This looseness is generated by the compression of the core due to voids in the core or the cusp like voids between the under side of helically applied wires and the cylindrical core assembly.
When load is applied to a cable either during manufacture or in use, the tension of the load bearing elements generate a pressure on the core tending to remove these voids. As the voids are removed by pressure, the effective core diameter is reduced and the helical elements must elongate to accommodate the smaller core diameter. This elongation or increases in length of the cable is a permanent or non elastic elongation of the cable and is very objectionable for applications where these cables are used for accurate length measurements.
A further problem associated with core compression occurs when the cable has more than one layer of load bearing elements. In this case the inner layer of wire will compress the core, but the subsequent layers will not necessarily adjust to this new diameter. The result is that the subsequent layers will possess residual stress causing loose wires, unbalanced torque and the like, which gives the cable unpredictable mechanical characteristics.
One common form of this type of cable is the cable used in oil well logging operations. This cable generally consists of one or more electrically insulated conductors covered with two layers of contra-helically wound wire of high strength steel. This type of cable is used to lower geophysical instruments and tools to depths of 30,000 feet and more and at temperatures up to 800.degree. F. To know the location of the instruments to a depth accuracy of one foot in 10,000 feet it is very important that the cable have elastic stretch characteristics and any permanent or irreversible changes in the cable length in use must be avoided. Because of the great depths and abrasive environment it is also very important that the cable does not have loose armor wires protruding which will wear more rapidly causing premature cable failure.
Consequently, all known manufacturing methods aim at the construction of armored cables which overcome the problems indicated.
For example, a measure often taken by cable manufacturers consist of subjecting the entire length of the finished cable before use, to a prestressing operation. In this operation the completed cable is tensioned and sometimes heated in an attempt to remove the inherent irreversible deformation characteristics normally experienced with this type of cable. However, this solution has not proved satisfactory, because after the prestressing operation the cable possesses stress and torsional unbalance, and in an effort to equalize this unbalance to cable rotates in the well. This rotation generates a permanent and irreversible elongation of the cable and causes inaccurate measurements of depth. Furthermore, the rotation is not determinable because it depends upon the depth and time the cable is left in the well, so that the cable in routine operations never fully stabilizes either its torque stress or its length. This rotation to equalize stress unbalance will also loosen armor wires.
Other known methods in the art deal with correcting the problems of compression of the core, loosening of armor and the residual torque stress, by applying heat, and tension and a rotation to the cables once they are completely assembled. However, all these known methods have not had the success desired, largely because the geometry of the multiple successive layers of armor wires is such that, when the core is compressed after all layers of wire are in place, it is impossible to eliminate all the residual tension and torque stress in each and every one of the layers of armor, without producing the great drawback above referred to of loose armor wires.